The manufacture of integrated circuits involves the printing of circuit patterns on a wafer of silicon. One method of printing the circuit patterns on a wafer involves coating the wafer with a photo-resist material. When exposed to light the photo-resist material polymerizes into a hard tenacious coating, whereas the unexposed portions of the layer are easily removed, e.g. by means of a solvent or developer. A glass or quartz disc which is opaque except for transparent areas comprising the circuit pattern is disposed in an optical arrangement for projecting the circuit pattern onto the wafer. In practice the circuit pattern is repeated on the mask in rows and columns such that each wafer contains a plurality of identical circuit patterns. Also circuit patterns on each wafer are built up layer by layer using a different mask for each layer until a complete circuit is formed. The wafer which contains a plurality of identical circuits is then cut along rows and columns producing a plurality of separate circuits or chips.
One system for projecting circuit patterns of a mask onto a wafer is disclosed in U.S. Pat. No. 4,011,011, entitled Optical Projections Apparatus, issued on Mar. 8, 1977, having the same assignee as the present application. This patent discloses the optical projection and alignment arrangement necessary to such a system.
In such a projection system alignment of the mask and wafer is critical. This is particularly true where layers of circuits are formed sequentially on a wafer. Errors of just a few microns may result in unaligned elements of a circuit element. The referenced patent overcomes to a large extent the problems of alignment.
However, it has been found that a mask cannot be perfectly aligned for projection onto a wafer without taking into account the change in size due to thermal expansion or contraction of the glass mask in the radial direction. While the glass disc may change dimension in the axial or thickness direction, such does not affect alignment.
The present invention contemplates a system for compensating for change in dimension of the mask due to temperature.